2.设计链表

707.设计链表

题目：

707. 设计链表 - 力扣（LeetCode）

代码：

能通过测试，但是提交不通过，不想改了。。。体会下虚拟头节点意思意思好了。

class ListNode{
    public ListNode next;
    public int val;
    public ListNode(int val, ListNode next){
        this.next = next;
        this.val = val; 
    }

    public ListNode(){}

    public ListNode(int val){
        this.val = val;
    }
}

class MyLinkedList {
    int size;
    ListNode head; // 虚拟头节点

    public MyLinkedList() {
        size = 0;
        this.head = new ListNode(0);
    }
    
    public int get(int index) {
        if(index >= size || index < 0){
            return -1;
        }
        ListNode cur = head;
        for(int i = 0; i <= index; i++){ // 由于包含了虚拟头节点，所以有等号
            cur = cur.next;
        }
        return cur.val;
    }
    
    public void addAtHead(int val) {
        head.next = new ListNode(val, head.next);
        size ++;
    }
    
    public void addAtTail(int val) {
        ListNode cur = head;
        while(cur.next != null){
            cur = cur.next;
        }
        cur.next = new ListNode(val);
        size ++;
    }
    
    public void addAtIndex(int index, int val) {
        ListNode cur = head;
        if(index < 0 || index > size){return;}
        if(index == size){
            addAtTail(val);
            size++;
            return;
        }
        for(int i = 0; i < index; i++){ // 包含虚拟头节点,但是不是找index而是找它的前置
            cur = cur.next;
        }
        cur.next = new ListNode(val, cur.next);
        size++;
    }
    
    public void deleteAtIndex(int index) {
        if(index >= size || index < 0){return;}
        ListNode cur = head;
        for(int i = 0; i < index; i++)
            cur = cur.next;
        cur.next = cur.next.next;
        size--;
    }
}

/**
 * Your MyLinkedList object will be instantiated and called as such:
 * MyLinkedList obj = new MyLinkedList();
 * int param_1 = obj.get(index);
 * obj.addAtHead(val);
 * obj.addAtTail(val);
 * obj.addAtIndex(index,val);
 * obj.deleteAtIndex(index);
 */